1. Field of the Invention
The present invention relates to a ferroelectric thin film, a substrate provided with a ferroelectric thin film, a device having a capacitor structure and a method for manufacturing a ferroelectric thin film. More particularly, the present invention relates to a ferroelectric thin film, a substrate provided with a ferroelectric thin film and a device having a capacitor structure used for a ferroelectric memory device, a pyroelectric sensor device, a piezoelectric device or the like, and a method for manufacturing a ferroelectric thin film.
2. Description of the Related Arts
Ferroelectrics have been widely used for the development of devices such as a condenser, an oscillator, an optical modulator and an infrared light sensor since ferroelectrics have numerous functions such as spontaneous polarization, high dielectric constant, electro-optical effect, piezoelectric effect and pyroelectric effect.
In accordance with the development of technique for forming a thin film, the application field of the ferroelectric thin film is becoming wider. For example, reduction of capacitor area for high integration of devices and improvement of reliability has been being developed by applying the high ferroelectric characteristics to various kinds of semiconductor devices such as a DRAM. Also, development of ferroelectric non-volatile memories (FRAM) having high density and high operation speed has been recently taking place by combination with semiconductor memory devices such as a DRAM. Ferroelectric non-volatile memories eliminate the need for back-up power supply by utilizing the ferroelectric properties (hysteresis effect) of the ferroelectrics. For the development of such devices, it is necessary to use a material having characteristics such as large remanent spontaneous polarization (Pr), small coercive field (Ec), small leakage currents and large resistance to repetition of polarization inversion. Further, it is desired to realize the above properties with a thin film having a thickness of 200 nm or below so as to reduce the operation voltage and to conform to fine processing of semiconductors.
For the purpose of applying thin films to FRAM or the like, forming of thin films with ferroelectric lead (Pb) oxide compounds such as PbTiO.sub.3, Pb(Zr.sub.1-x Ti.sub.x)O.sub.3 (PZT), PLZT is now being tried by employing methods for forming a thin film such as sputtering, vapor deposition, sol-gel method and MOCVD method.
Among the above-described ferroelectric materials, Pb(Zr.sub.1-x T.sub.x)O.sub.3 (PZT) is the one that has been intensively studied recently, and a thin film having a good ferroelectric property has been obtained by sputtering method or sol-gel method. For example, a thin film having a remanent spontaneous polarization Pr as large as 10 .mu.C/cm.sup.2 to 26 .mu.C/cm.sup.2 has been obtained. However, there is a problem that leakage currents occur and resistance to polarization inversion decrease as the thickness of the film is reduced (thinning of the film). This is due to the fact that the ferroelectric property of PZT, which depends largely on the composition x, is liable to change because PZT contains Pb having a high vapor pressure and, therefore, the film composition is liable to change at the time of forming or heat-treatment of the film, and also due to generation of pinholes and generation of layers having a low dielectric constant owing to the reaction of the underlying layer and Pb. Accordingly, the development of other materials excellent in ferroelectric property and resistance to polarization inversion is desired. Also, in view of the application to integrated devices, high density of the thin film is required for fine processing of the devices.
As other oxide ferroelectrics, there is known a group of bismuth oxide ferroelectrics having a layered crystal structure such as shown by the following general formula. EQU Bi.sub.2 A.sub.m-1 B.sub.m O.sub.3m+3,
wherein A is selected from Na.sup.1+, K.sup.1+, Pb.sup.2+, Ca.sup.2+, Sr.sup.2+, Ba.sup.2+ and Bi.sup.3+ ; and B is selected from Fe.sup.3+, Ti.sup.4+, Nb.sup.5+, Ta.sup.5+, W.sup.6+ and Mo.sup.6+ ; and m is a positive integer. The basic crystal structure of the ferroelectrics represented by the above general formula is such that a layered perovskite layer consisting of a series of perovskite lattices made of (m-1) ABO.sub.3 is interposed between (Bi.sub.2 O.sub.2).sup.2+ layers. Numerous materials in which A is selected from Sr, Ba and Bi, and B is selected from Ti, Ta and Nb show ferroelectric properties.
Among the ferroelectrics represented by the above general formula, Bi.sub.4 Ti.sub.3 O.sub.12 (bismuth titanate) is a ferroelectric having a layered perovskite structure (rhombic crystal/lattice constants: a=5.411 .ANG., c=32.83 .ANG.) with strong anisotropy. The ferroelectric property of its single crystal is such that, along the a-axis, the remanent spontaneous polarization is Pr=50 .mu.C/cm.sup.2 and the coercive field is Ec=50 kV/cm; and, along the c-axis, the remanent spontaneous polarization is Pr=4 .mu.C/cm.sup.2 and the coercive field is Ec=4 kV/cm. As shown above, Bi.sub.4 Ti.sub.3 O.sub.12 has the strongest a-axis component of spontaneous polarization and has a very small c-axis component of the coercive field when compared with the above other bismuth oxide ferroelectrics.
It will be possible to apply this ferroelectric to electronic devices such as ferroelectric non-volatile memories if the orientation of thin films can be controlled for utilizing the properties of large spontaneous polarization and small coercive field which Bi.sub.4 Ti.sub.3 O12 has. However, the cases that have been reported so far utilize only the c-axis orientation along which the spontaneous polarization is small or the random orientation, so that the large spontaneous polarization along the a-axis has not been utilized to the full extent yet.
On the other hand, formation of a thin film with Bi.sub.4 Ti.sub.3 O12 has so far been tried by MOCVD method or sol-gel method. Among these, the conventional sol-gel method of forming a ferroelectric thin film need thermal treatment processing of 650.degree. C. or more for obtaining a good ferroelectric property and, further, it has been difficult to apply the thin film to highly integrated devices which need fine processing because the film surface morphology consists of crystal particles of about 0.5 .mu.m. Moreover, since the Bi.sub.4 Ti.sub.3 O.sub.12 thin film of the c-axis orientation is formed on a Pt electrode layer/SiO.sub.2 insulation film/Si substrate or on a Pt substrate with the substrate temperature being 600.degree. C. or more, the formation of ferroelectric thin films by MOCVD method cannot be directly applied to actual device structures. Namely, as in the case of Pt/SiO.sub.2 /Si substrate, a bonding layer such as a Ti film must be formed between the Pt electrode layer and the underlying SiO.sub.2 film in order to ensure the bonding strength between the Pt electrode layer and the SiO.sub.2 film. However, it has been reported that, if a Bi.sub.4 Ti.sub.3 O12 thin film is formed by MOCVD method on the Pt electrode substrate having the bonding layer, the film surface morphology consists of gross crystal particles and pyrochlore phase (Bi.sub.2 Ti.sub.2 O.sub.7) is prone to be generated (See Jpn, J. Appl. Phys., 32, 1993, pp. 4086, and J. Ceramic Soc. Japan, 102, 1994, pp. 512). If the film surface morphology consists of gross crystal particles, the thin film cannot be applied to highly integrated devices which needs fine processing and, moreover, pinholes are generated by thin films, causing leakage currents. Accordingly, it is difficult to form a ferroelectric thin film having a good ferroelectric property with a thickness of 200 nm or less by such a conventional technique.
As described above, the conventional technique involves problems that the large spontaneous polarization of Bi.sub.4 Ti.sub.3 O.sub.12 along the a-axis has not been utilized to the full extent and that the density and the flatness of the film surface that are needed for fine processing and reducing leakage currents in view of applying the ferroelectric thin film to highly integrated devices have not been obtained.